This invention pertains to a system for coupling signals to a transmitting antenna from a source of television video signals and an associated source of audio signals in which separate visual and aural power signals are diplexed together for application to the antenna but in which the aural signal can be multiplexed with the visual carrier and switched through the aural power amplifier in the event of failure of the visual power amplifier.
Broadcast television transmitters are used to generate high-power modulated signals for application to broadcast antennas. Such transmitters for television use may be capable of generating hundreds of kilowatts of visual signal energy within the television band. The aural carrier signal power is typically about 10 dB down from the peak visual carrier energy.
In one form of transmitter, the aural carrier is FM-modulated onto a 4.5 MHz carrier. The FM-modulated aural carrier is combined with baseband video, and the combined signal is amplitude-modulated onto the final carrier. Amplifiers are used to raise the power level of the combined visual-aural signal to the desired level. Another form of transmitter AM-modulates the visual carrier onto an IF signal and FM-modulates the aural carrier onto an IF signal displaced in frequency from the first IF signal by the desired final difference between visual and aural carriers, combines the two signals, AM modulates the combined signal, and amplifies the composite signal. In yet another type of transmitter, a high-power oscillator tube amplitude modulates the combined visual and FM-modulated aural signals. All of these configurations have combined visual and aural signals passing through a high-power active device. As is well known, active devices are subject to nonlinearities of both amplitude and phase which lead to a multitude of undesirable intermodulation products. These products must be kept to a low level either by reducing the power output of the active device or by filtration. Filtration, however, cannot adapt to the various intermodulation products, which vary with signal level. As a result, the transmitter arrangements in which both aural and visual signals pass through power active devices tend to be limited in power output.
It is known to overcome the above-mentioned limitations by keeping the visual and aural carriers separate through the active devices, and then combine the two separate high-power signals in a passive linear combiner. In such diplexed arrangements, one active device (or one set of active devices) produces the high-power modulated visual carrier and another produces the high-power aural signal. These are combined by a linear diplexer. A known form of diplexer consists of two 3 dB, 90.degree. four-port couplers, two ports of each of which are coupled together by transmission lines. A further port of each hybrid is coupled to a source of high-power carriers (one to the visual, one to the aural). On the hybrid, to which the visual carrier is coupled, the fourth port is terminated. On the hybrid, to which the aural carrier is coupled, the fourth port is the combined output port at which the linearly combined visual and aural carriers appear, and this port is normally coupled to the antenna. The transmission lines, coupling together two ports of each hybrid, are also coupled to transmissionline filters, including one tuned to 3.58 MHz below the visual carrier for preventing the lower-sideband visual-carrier-color-subcarrier intermodulation product from reaching the antenna, and another tuned to the aural carrier frequency for causing the reflections necessary to couple the aural carrier input port of the second hybrid to the antenna port. In such diplexers for UHF TV use manufactured by Micro Communications, Inc., located at Grenier Field, Manchester, N.H., the transmission-line filters are short-circuited circular waveguide. In another such diplexer manufactured by RCA Corporation, Gibbsboro, New Jersey, the transmission-line filters are short-circuited rectangular waveguide.
For television transmitters of the diplexed type, if the high-power aural carrier active device or aural power amplifier fails, transmission of the visual carrier without aural carrier results. To overcome this situation, it is desirable to be able to revert to the less desirable multiplexed operation from the more desirable diplexed operation in the event of failure of the aural power amplifier. In the past, this has been accomplished by coupling the low-level FM-modulated aural carrier with low-level visual information and applying the combination to the visual power amplifier, and at the same time bypassing the diplexer by means of high-power switch circuits. The low-level combining circuits are not a great problem, but the switching of the high-power signal requires very elaborate and expensive switches and circuits, or temporary shutdown of the still-operating high-power visual sections so as to be able to throw switches. U.S. Pat. No. 4,491,871 which issued to A. N. Schmitz and R. N. Clark on Jan. 1, 1985, describes an arrangement in which the aural signal can be multiplexed without high-power switching in the event of failure of the aural power amplifier by simultaneously detuning the aural cavities in the diplexer in order to allow the aural-multiplexed visual carrier to pass through the diplexer.
If the visual power amplifier fails, transmission of the aural carrier without visual carrier results. To overcome this situation, it is desirable to be able to revert to the multiplexed operation in the event of failure of the visual power amplifier, whereby the aural power amplifier can be substituted for visual service. However, wheh using the aural power amplifier for visual service, the rf (radio frequency) energy must be switched to bypass the antenna diplexer. The present invention provides an arrangement in which the aural power amplifier can be used for visual service without high-power switching circuitry.